You need to edit the config.xml in the relevant folder.
You go in the game to preferences, show unique IDs, see what ID belongs to the item you want to use the custom content to.
You put the resource you want with that number as filename.
Then open the config.xml in the folder, scroll down, see what's the structure of it and add the line for your item. Surely you can copy one of the others and change the other number with yours. Save and done.